On Monday, I finally ran an encounter in which the party fought and slew the Tarrasque.
Ever since I first cracked open the Monster Manual in 2015 and beheld the mightiest of its monsters, I wanted to have an epic encounter with the thing. But at CR 30, according to all encounter-building guidance, there's basically no level at which it's not an extremely deadly encounter, at least officially.
In practice, though, a party of 7 18th-level adventurers (6 PCs and one statted-out DMPC) with a lot of powerful magic items was enough to do it.
Let me set the scene:
I'm running a campaign that is set in the Magic multiverse, primarily on Ravnica (all the PCs are from there, and most of the campaign has been set on that city-plane). However, in order to stop the Phyrexians from circumventing the Mending-era restriction on interplanar travel for non-Planeswalkers (I structured this campaign before the whole March of the Machines plot,) the party (all planeswalkers) need to reassemble the Golgothian Sylex to destroy the Cosmic Key, an colossal artifact that the Phyrexians have built through the nine layers of New Phyrexia. The Cosmic Key would allow them to essentially take over a number of planar wormholes created by the Ilithid to move between the Magic multiverse (aka Dominia) and the Great Wheel cosmos of D&D. If this artifact is completed, the Phyrexians would be able to launch a multiverse-wide invasion (actually very similar to what they do with Worldbreaker in the canon story that I swear had not come out yet when I came up with this plot).
In my campaign, after the destruction of Terisiare, Urza was able to break the Golgothian Sylex into six shards and hid them across the multiverse on different planes: Theros, Innistrad, Arcavios, Ikoria, Eldraine, and New Capenna.
After getting the shards from Theros, Innistrad, and Arcavios, the party went to find the one on Ikoria, which had been placed in the belly of the mightiest of its behemoth-titans, the Tarrasque.
Thus, the party had to hunt across the plane for the titan, eventually tracking it down in the region of Ketria.
I used the 5.5/2025 version of the Tarrasque. This had benefits and downsides: the benefit was that the Thunderous Bellow is a very powerful AoE spell that inflicted incredible damage on the party. The downside is that the Reflective Carapace is less powerful, only deflecting specifically spells that use attack rolls or magic missile. This allowed them to use Lightning Bolt to do some damage to it (though I had misremembered and thought this would also work on Disintegrate or Finger of Death).
I had two basic win conditions: the party could either kill the Tarrasque and find the shard within its remains, or they could allow one of their number to be swallowed, make a desperate search for the shard inside, and then try to get out.
They had some advantages: the Barbarian had Freedom of Movement cast on him - while this only prevents being restrained magically, it still allows you to escape nonmagical restraints and grapples using 5 feet of movement. While the Tarrasque prevents teleportation from within its stomach, this was the ace that allowed the Barbarian to exit as soon as he was able to find the shard.
Another big advantage was Primal Knowledge. While both blinded and restrained inside the belly, Primal Knowledge allowed the Barbarian to use Strength for his Perception check, which in turn meant advantage to cancel out the disadvantage for being blinded.
The check to find the shard within the belly was a DC 30 Perception check, and the Barbarian happened to roll 19 on the die, with a +5 to Strength and a +6 proficiency bonus, which you'll note amounts to exactly what he needed.
At this point, the party had actually done about 600 of the Tarrasque's just-under 700 HP. An AC of 25 is the highest in the game (though Sul Khatesh has infinite uses of the Shield spell, so in theory she could always have an AC of 27) but when you have players with +14 to hit, that's actually not that difficult to hit (one of the two Storm Sorcerers had a Potion of Cloud Giant strength and a Vorpal Greatsword - giving him a +11 to hit normally and then, Wishing for Tenser's Transformation, he also got proficiency, putting that at a +17 to hit and also automatic advantage).
If I made a single error in running as brutally as possible, it's that I skipped the legendary action World Shaking Movement to break the sorcerer out of Tenser's Transformation - this ultimately proved fatal to the Tarrasque, as the final blow was a critical hit with all the bells and whistles of both Tenser's Transformation and a Vorpal Sword by said Sorcerer.
The strongest card for the Tarrasque was its Thunderous Bellow. While the party had a Heroes' Feast that prevented the Frightened condition the ability imposes, the sheer amount of damage (12d12 Thunder, for an average of 78) and a Con save DC of 27 (between Sorcerers, Barbarians, and Artificers in the party, there were a lot of Con-proficient folks, but this is still a hefty DC to hit,) this was enough to leave half the party unconscious by the end of the fight.
I did have to make an alteration to the stat block, though: because the party has two Storm Sorcerers, in theory the party could have merely had the two of them attack from far outside the Tarrasque's melee range and bombard it without fear because of their full immunity to Thunder damage.
However, Ikoria has a canonical tendency for its monsters to mutate when exposed to the world's strange crystals, so while I had its first bellow do Thunder damage, on its second turn, I had it bite into a piece of crystal (using one of its attacks,) which then changed the Bellow's damage type to Force (I flavored this as similar to when, in Godzilla Minus One, good old Gojira's spinal plates light up with pink energy before it does its atomic breath. Here, the glow was blue).
I did actually recharge the bellow twice (it's a 5-6 recharge, so this was fairly lucky).
The fight ended during round four (not counting a couple of death saves). Indeed, while the shard had been secured before the Tarrasque was dead, because people were down, the party couldn't do the easy thing and just planeswalk away (in my game, they can basically use a once-a-day plane shift that only works for themselves, and starts getting messy if they're going to a plane they haven't been to before).
What's kind of fascinating about the Tarrasque on a mechanical level is that it's actually pretty simple to run. I'm looking forward to breaking out the 5.5 version of the Lich (which will be, among a lot of very powerful minions including purple worms and planetars, the boss that gets them to level 20) but I suspect that will be a complex fight to run.
The Tarrasque isn't that complex - there's a pretty clear decision tree for it. While it does have things like its Reflective Carapace and its World-Shaking Movement that help the DM shut down some of the most powerful player abilities, especially for spellcasters (who tend to get super-powered at these late levels), the real threat of the Tarrasque is that it just pours out damage like crazy and has a ton of HP. With a +19 to hit, I think I only missed maybe one attack, when I got a natural 2 and the Sorcerer popped a shield spell (being a Loxodon, they have a pretty high base AC).
Ironically, the party grew frustrated that the Tarrasque wasn't biting and swallowing the Barbarian. This was simply because I kept recharging Thunderous Bellow - the one turn that it did its full multiattack, it had to use its bite to get the crystal to change its bellow to Force damage (it had recharged it on that turn, but I wanted to give the Sorcerers something to fear).
I think the one player who really felt shut down was the Artillerist - their cannons were able to shoot the Tarrasque (and in fact, reading the 2014 version for this post, they'd have actually worked on that one as well) but the Thunderous Bellow specifically also hits Objects, and on top of that, the Tarrasque is a siege monster and thus does double damage against objects and structures, so those poor things each took like 172 damage right after only being out for a round.
The clutch move at the end (which thankfully didn't rob me of a full turn, but did prevent some legendary actions) was a Power Word Stun by the other Storm Sorcerer, which gave a bit of breathing room to start healing the downed people. While the Tarrasque still had three of its six legendary resistances left, the initial stun from a Power Word spell doesn't involve a saving throw, and the Tarrasque is not immune to that condition.
In all honesty, this is the kind of single monster encounter that I've sort of felt unable to challenge people with for a while in this campaign. While I had to roll an ungodly number of dice, it was something of a relief to have such a simple encounter.
